Issue with subtitle size

I transcode my movies and convert the PGS subtitle files to srt files. Whenever I watch one of my movies on an iPhone, iPad or AppleTV everything is fine but as soon as I open up the web browser or an NVIDIA Shield, I get these enormous subtitles and I have no idea how I can get rid of this issue for once and for all.
I usually convert my video to mp4 (m4v) and add the subtitles with Subler.

Hope someone here can help me out with this.

Why are you using the web browser on your Shield when there is a native Plex app available for it?

It is the native app, on all devices but the issue also exists on web browser

And what happens if you use this very same web browser to go to
Settings - Web - Player - ‘Subtitle Size’
and try the other available sizes?

A different strategy: don’t add the subtitle into the file.
Just put the SRT file beside the video file and name it exactly alike, only difference being the file name extension.
https://support.plex.tv/hc/en-us/articles/200471133-Adding-Local-Subtitles-to-Your-Media

Nothing and like I said, I’m not using the browser, but I know it happens on the browser too. Only iOS and TV look immune

Have you tested my other suggestion?

Not yet, will try it today and keep you posted!